A body implantable lead forms an electrical connection between a patient's anatomy and a pulse generator such as a cardiac pacemaker, an implantable cardioverter defibrillator (ICD), an appetite or pain suppression device, and the like. The lead includes a lead body comprising a tubular, flexible biocompatible, biostable insulative sheath or housing, such as formed of silicone rubber, polyurethane or other suitable polymer. One example of a lead body is a bipolar lead having a tip electrode and a ring sensing electrode. Generally bipolar leads include two coaxial conductors with insulation therebetween that are carried within the insulative housing. Another example of a lead body is a cardioverter/defibrillator lead that includes a sensing ring, a shocking right ventricle (RV) electrode, a shocking superior vena cava (SVC) electrode and a tip sensing/pacing electrode. The lead includes a multi-lumen housing, each lumen of which carries a separate conductor through the lead housing to each of the sensing ring, RV electrode, SVC electrode and tip electrode.
Magnetic resonance imaging (MRI) is commonly used as an efficient technique in the diagnosis of many injuries and disorders. MRI scanners provide a non-invasive method for the examination of internal structure and function. The MRI scanner includes a static magnetic field, a gradient magnetic field and a radio frequency (RF) magnetic field. The static magnetic field aligns protons of hydrogen atoms in the body, while the RF magnetic fields expose the protons to varying RF fields which cause the protons to spin and thus produce a faint signal that is detected by a receiver portion of the MRI scanner. The static magnetic field may have a field strength of between 0.2 and 3.0 Tesla. A nominal value of 1.5 Tesla is approximately equal to 15,000 Gauss. The time varying or gradient magnetic field may have a maximum strength of approximately 40 milli-Tesla/meters at a frequency of 0 5 KHz. The RF magnetic field may have a frequency between 8 and 215 MHz. For example, up to 20,000 watts may be produced at 64 MHz in a static magnetic field of 1.5 Tesla.
A concern has arisen regarding the potential interaction between the MRI environment and implantable leads and devices. In particular, implantable leads may experience RF-induced current. The RF induced current has been found to raise the temperature in the leads by 25 degree Centigrade or higher.
Heretofore, leads have been proposed for use with MRI-safe implantable medical devices. These proposed leads are coupled to, or have housed therein, a discrete resonant tuning module. The resonant tuning module includes a control circuit for determining a resonance frequency of the implantable device and an adjustable impedance circuit to change the combined resonant frequency of the medical device and the lead. The resonant circuit includes an inductor (L) coupled in parallel with a capacitor (C) to form a discrete LC circuit. The inductance and capacitance values of the inductor and capacitor are tuned approximately to the frequency of an expected RF magnetic field in an MRI scanner.
However, it remains challenging to implement discrete LC and L circuits within leads while still meeting performance requirements. For example, circuit size is a challenge as there is a continued desire to provide circuits that are small enough to be packaged inside the distal portion of a lead. Also, it is difficult to locate a discrete LC or L circuit in the distal or proximal end of a lead without changing the internal design of existing leads. Further, to implement the LC or L circuits in pacemakers, the circuit should be able to withstand large surges of energy such as when a shock is delivered by an external defibrillation. Other challenges include sensitivity to design variables in leads (e.g., shielding effect; subtle changes around the component may alter heating significantly), cost, and biocompatibility (e.g., Hermetic seal; Mechanical requirements).
It would be desirable to provide an implantable medical lead that is safely operated in an MRI environment. It would be further desirable to provide an improved implantable medical lead that may be operated in an MRI environment without the generation of significant heat in the lead. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.